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This is patch that adds support for interruptible FFI calls in the form
of a new foreign import keyword 'interruptible', which can be used
instead of 'safe' or 'unsafe'.  Interruptible FFI calls act like safe
FFI calls, except that the worker thread they run on may be interrupted.

Internally, it replaces BlockedOnCCall_NoUnblockEx with
BlockedOnCCall_Interruptible, and changes the behavior of the RTS
to not modify the TSO_ flags on the event of an FFI call from
a thread that was interruptible.  It also modifies the bytecode
format for foreign call, adding an extra Word16 to indicate
interruptibility.

The semantics of interruption vary from platform to platform, but the
intent is that any blocking system calls are aborted with an error code.
This is most useful for making function calls to system library
functions that support interrupting.  There is no support for pre-Vista
Windows.

There is a partner testsuite patch which adds several tests for this
functionality.

and remove the temporary DOpt class workaround.

Implements Trac #4299.  Documentation to come.

This fixes trac bug #1344

This also means that extsBitmap gets set, whereas is was just being set
to 0 before.

Now not only is if/then a bracketting pair, but then/else is too

If enabled, these accept more layout, but give warnings

It used to break on
    let {x = 'a'} in x
as the 'in' token would keep closing contexts looking for an implicit
'let' layout.

After some discussion we decided to make a quasi-quote look like

   [pads| ...blah... |]

rather than

   [$pads| ...blah... |]

as before. The new syntax is quieter, although it does not signal
quite as clearly that there is a splice going on.

It thought the ) needed to close something, but the $( hadn't
opened anything.

The problem in #3741 was that we had confused column numbers with byte
offsets, which fails in the case of UTF-8 (amongst other things).
Fortunately we're tracking correct column offsets now, so we didn't
have to make a calculation based on a byte offset.  I got rid of two
fields from the PState (last_line_len and last_offs).and one field
from the AI (alex input) constructor.

Also corrected a couple of line 0's to line 1

And make the code simpler in the process!

Caution: Largely untested

The type of the CmmLabel ctor is now
  CmmLabel :: PackageId -> FastString -> CmmLabelInfo -> CLabel
  
 - When you construct a CmmLabel you have to explicitly say what
   package it is in. Many of these will just use rtsPackageId, but
   I've left it this way to remind people not to pretend labels are
   in the RTS package when they're not. 
   
 - When parsing a Cmm file, labels that are not defined in the 
   current file are assumed to be in the RTS package. 
   
   Labels imported like
      import label
   are assumed to be in a generic "foreign" package, which is different
   from the current one.
   
   Labels imported like
      import "package-name" label
   are marked as coming from the named package.
   
   This last one is needed for the integer-gmp library as we want to
   refer to labels that are not in the same compilation unit, but
   are in the same non-rts package.
   
   This should help remove the nasty #ifdef __PIC__ stuff from
   integer-gmp/cbits/gmp-wrappers.cmm